Fuel assembly for a nuclear reactor

ABSTRACT

A fuel assembly for a boiling water reactor comprising at least one rotary cell ( 13   a   -13   d ) which has fuel rods ( 4 ) arranged in a number of concentric rings with a substantially circular shape, and a steam conducting channel ( 17   a   -17   d ) arranged in the centre of the concentric rings through which steam flows upwards through the fuel assembly. At least certain of the fuel rods in the rings are arranged such that their upper ends are displaced in relation to their lower ends in the tangential direction such that water and steam are brought to rotate around the steam conducting channel.

TECHNICAL FIELD

The present invention relates to a fuel assembly for a boiling waterreactor which is adapted, during operation of the reactor, to allowcooling water to flow upwards through the fuel assembly while absorbingheat from a plurality of fuel rods which are surrounded by a fuelchannel, whereby part of the cooling water is transformed into steam,and where the fuel assembly comprises a steam conducting channel throughwhich the steam is allowed to flow through the fuel assembly towards theoutlet end thereof.

BACKGROUND ART

In a boiling water nuclear reactor, moderated by light water, the fuelexists in the form of fuel rods arranged in a certain, normallysymmetrical pattern, a so-called lattice, and is retained at the top bya top tie plate and at the bottom by a bottom tie plate. A fuel assemblycomprises one or more bundles of fuel rods which are surrounded by afuel channel with a substantially square cross section. In the core ofthe reactor, the fuel assemblies are arranged vertically and spaced fromeach other. During operation, the water is admitted through the bottomof the fuel assembly and then flows upwards through the fuel assemblypast the fuel rods. The heat emitted by the fuel rods is taken up by thewater which starts boiling, whereby part of the water is transformedinto steam. The water and the steam are passed out through the upper endof the fuel assembly. The produced steam is delivered to turbines whichdrive generators where electrical energy is generated.

A disadvantage with a boiling reactor is the high proportion of steam byvolume in the upper part of the fuel assembly.

When the proportion of steam by volume rises in the coolant, its abilityto carry off heat from the fuel rods is reduced, thus increasing therisk of dryout, which in turn leads to an increase of the risk of fueldamage.

Still another problem with a high steam volume in the fuel is that steamis inferior to water as moderator, which results in the moderation beinginsufficient whereby the fuel is utilized inefficiently. In the lowerpart of the fuel assembly, the moderator consists of water whereas themoderator in the upper part of the fuel assembly consists of both steamand water. This means that the fuel in the upper part of the fuelassembly cannot be utilized efficiently. It is, therefore, desirable tokeep down the steam volume in the coolant while at the same timemaintaining the steam generation at a high level.

The faster the steam disappears out from the fuel assembly, the lowerthe steam volume. A separation of the steam flow and the water flow inthe upper part of the fuel assembly thus gives the steam flow a highervelocity than the water flow, whereby the proportion of steam by volumein the fuel assembly is reduced. In this way, the margin with respect todryout is improved and the fuel in the upper part of the fuel assemblyis utilized in a better way.

U.S. Pat. No. 5,091,146 discloses a fuel assembly which attempts toachieve a separation of the steam flow and the water flow in the upperpart of the fuel assembly by arranging a steam pipe above one or morepart-length fuel rods, that is, fuel rods extending from the bottom tieplate but terminating below and at a distance from the top tie plate. Inthis way, the steam which is generated in the coolant is to be diverted.The pipe has openings both in its upper and its lower end. Thedisadvantages of such a pipe are several. For one thing, it may beexpensive to manufacture, and, for another, it gives an increasedpressure drop in the upper part of the fuel assembly. Anotherdisadvantage is that it may be difficult to cause the continuouslyproduced steam to enter the pipe. Admittedly, the pipe is provided withopenings and other devices to cause the steam to flow into the pipe andto prevent water from entering the pipe, but it is still doubtfulwhether this is an effective way of causing the steam to enter the tube.

SUMMARY OF THE INVENTION

The object of the invention is to provide a fuel assembly which in asimple and efficient way separates the steam flow and the water flow atleast partially, thus obtaining a lower proportion of steam by volume inthe fuel assembly.

What characterizes a fuel assembly according to the invention willbecome clear from the appended claims.

A fuel assembly according to the invention comprises a vertical channelwhich conducts steam upwards through the fuel assembly during operationof the reactor. This channel has no walls but only comprises an emptyvolume between the fuel rods and will hereinafter be referred to as asteam conducting channel. The fuel assembly is designed such that thecoolant, that is, water and steam, is caused to rotate around the steamconducting channel so as to form an upward eddy. The eddy rotates sofast that the steam separates from the water with the aid of thecentrifugal force. The water, which is heavier than the steam, is thrownoutwards and away from the steam conducting channel, whereas the lightersteam is pressed against the centre of the eddy and hence against thesteam conducting channel. This gives the steam a considerably higherspeed than the natural speed and the steam is able to leave the fuelassembly, at a high speed, via the steam conducting channel. In thisway, the proportion of steam by volume in the fuel assembly is reduced.

To achieve a rotation of water and steam around the steam conductingchannel, this channel is surrounded by fuel rods arranged in concentricrings with a substantially circular shape. The steam conducting channelis arranged in the centre of these rings. The fuel rods in the rings arearranged such that their upper ends are displaced in a tangentialdirection in relation to their lower ends, so as to form a helix. Inthis way, the coolant is forced to rotate around the steam conductingchannel while at the same time moving upwards through the fuel assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a vertical section G—G through a firstembodiment of a fuel assembly according to the invention.

FIG. 2a shows a horizontal section A—A through the lower part of thefuel assembly in FIG. 1.

FIG. 2b shows a horizontal section B—B through the upper part of thefuel assembly in FIG. 1.

FIG. 2c shows an alternative embodiment of the lower part of the fuelassembly in FIG. 1.

FIG. 3 shows how the fuel rods in a rotary cell are arranged inclined ina tangential direction.

FIG. 4a schematically shows a vertical section H—H through a secondembodiment of a fuel assembly according to the invention.

FIGS. 4b-4 c show two horizontal sections C—C and D—D through the fuelassembly in FIG. 4a.

FIG. 4d shows for the fuel assembly in FIG. 4a how the lattice in anupper fuel unit is displaced outwards in relation to the lattice in alower fuel unit.

FIG. 5 schematically shows a vertical section J—J through a thirdembodiment of a fuel assembly according to the invention.

FIG. 5a shows for the fuel assembly in FIG. 5 how the lattice in anupper fuel unit is displaced outwards in relation to the lattice in alower fuel unit.

FIG. 6 shows a horizontal section F—F through the fuel assembly in FIG.5.

FIGS. 7a-7 c show a guide bar.

FIG. 8a shows an end plate with rotary vanes. FIG. 8b shows the shape ofthe end plate in more detail.

FIG. 9 schematically shows a fuel assembly according to one embodimentof the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

To be able to design the fuel assembly such that water and steam areseparated in an efficient way, it is desirable that the fuel assembly isso flexible that it may be given different shapes in the axial andradial directions in a simple way. Such a fuel assembly is shown inPCT/SE95/01478. This fuel assembly comprises a plurality of fuel unitsstacked on top of each other, each of which contains a plurality of fuelrods extending between a top tie plate and a bottom tie plate. The fuelunits are surrounded by a common fuel channel of substantially squarecross section. A fuel assembly of this type may, in a simple manner, begiven axially and radially different shapes.

Swedish patent application No. 9602447-6, which is still unpublished atthe time of filing of the present application, shows a fuel assemblywhich has a steam conducting channel in the form of an empty volumeextending through part of the fuel assembly, and which is surrounded byfuel rods arranged in concentric rings with a substantially squareshape. The fuel rods are arranged in an orthogonal lattice where eachfuel rod is included in two rows perpendicular to each other. The fuelrods in the rings are arranged such that the upper attachment point isdisplaced in relation to the lower attachment point in a tangentialdirection, such that water and steam are caused to rotate around thesteam conducting channel. The fuel assembly comprises a plurality offuel units stacked on top of each other, each of which contains aplurality of fuel rods extending between a top tie plate and a bottomtie plate. One disadvantage of this fuel assembly is that it isdifficult to obtain sufficient rotation of the water such that water andsteam are separated. Another disadvantage is that it is difficult toincline the fuel rods in the corners of the square rings without thefuel rods being brought into contact with each other.

To solve the above-mentioned problems, this application suggests a fuelassembly in which the fuel rods are instead arranged in a polar lattice,that is, fuel rods are arranged as substantially circular and concentricrings, and the steam conducting channel is arranged in a centre of theserings. This gives a lattice which has a symmetry corresponding to thedesired rotation. Fuel rods within the same ring have largely the samedistance to the centre of the steam conducting channel. A steamconducting channel surrounded by fuel rods arranged in concentric ringswith a substantially circular shape will hereinafter be referred to as arotary cell. A fuel assembly according to the invention has fourquadrants, each of which comprises one rotary cell. To further guide theflow around the steam conducting channel, each rotary cell is at leastpartially surrounded by a flow-guiding structure.

FIGS. 1, 2 a and 2 b show a fuel assembly according to the invention.During operation, the fuel assembly is arranged vertically in thereactor core. FIG. 1 shows a vertical section G—G through the fuelassembly. FIG. 2a shows a horizontal section A—A through the lower partof the fuel assembly and FIG. 2b shows a horizontal section through theupper part of the fuel assembly. The fuel assembly comprises an upperhandle 1, a lower end portion 2 and a plurality of fuel units 3 a and 3b, stacked on top of each other. Each fuel unit comprises a plurality offuel rods 4 arranged between the a top tie plate 5 and a bottom tieplate 6. The fuel units a re stacked on top of each other in thelongitudinal direction of the fuel assembly and they are stacked in sucha way that the top tie plate 5 in one fuel unit is facing the bottom tieplate 6 in the next fuel unit in the stack. A fuel rod 4 comprises fuelin the form of a stack of uranium pellets arranged in a cladding tube.The fuel assembly is enclosed in a fuel channel 7 of substantally squarec ross section. In this embodiment, the fuel assembly comprises eightfuel units, each having a height of 0.5 m.

The fuel units are suspended from and may be lifted with a centre tube 8a which is arranged centrally in the fuel assembly. The centre tubesurrounds a flow channel 8 b in which non-boiling water flows throughthe fuel assembly to improve the moderation in its central parts. Thecentre tube also con stitutes part of the flow-guiding structure whichsurrounds each rotary cell and is designed such that its walls guide thecoolant flow so as to become deflected around the rotary cells. Thecentre tube in FIGS. 2a and 2 b substantially have a square shape withconcave walls.

In the longitudinal direction of the fuel assembly there are fourV-shaped guide bars 9 a which are attached to the fuel channel 7. Theguide bars are arranged between the rotary cells and have two functions.For one thing, they constitute a stiffening of the fuel channel which inthis way may be made with relatively thin walls, and, for another, theyconstitute part of the flow-guiding structure. The guide bars in FIGS.2a and 2 b have concave walls which conduct the coolant flow in therotation around the steam conducting channels. In this embodiment, theguide bars extend through largely the whole fuel assembly and have asubstantially constant cross-section area. The guide bars 9 a form,together with the fuel channel 7, four bypass channels 9 b which conductwater along the walls of the fuel channel. It may be advantageous toallow a certain amount of steam formation in the bypass channels forspectral shift effect.

FIG. 7a shows in more detail a possible embodiment of a guide bar. Theguide bar is provided with elongated, oblique indentations 10 whichcontribute to the flow rotation and, in addition, give a mechanicalstiffening. Alternatively, flow-guiding bulges may be made. The portionsof the guide bar which support the top tie plates and the bottom tieplates at the joints between the fuel units have no indentations. Theguide bar is attached to the fuel channel by means of spot welds. FIG.7b shows a section L—L through the guide bar in FIG. 7a. FIG. 7c shows afolded-in portion 11 in the upper end of the guide bar for guiding thefuel units. The corners 7 b of the fuel channel are rounded andconstitute part of the flow-guiding structure. Welded corner bars 12provide additional reinforcement of the fuel channel and may, inaddition, serve as a support adapted to any detector probes between thefuel assemblies in the core.

FIG. 2a shows a fuel unit comprising four rotary cells 13 a, 13 b, 13 c,13 d. In the figures, the direction of rotation of the coolant flow inthe rotary cells are marked with arrows. In two of the rotary cells, 13b, 13 d, the coolant flow rotates in an anticlockwise direction and inthe other, 13 a, 13 c, the coolant flow rotates in a clockwisedirection. The rotary cells each have 28 fuel rod positions arranged ina polar lattice. A fuel rod position is a position in the lattice whereit is possible to arrange a fuel rod, but all the positions in thelattice need not be occupied by fuel rods. The polar lattice comprisesthree circular concentric rings. The inner ring has four positions, themiddle ring has eight positions, and the outer ring has 16 positions. Afuel rod in a fuel unit has its lower end arranged in a first fuel rodposition in the bottom tie plate and its upper end arranged in a secondfuel rod position in the top tie plate. By arranging the upper and lowerends of the fuel rod in separate fuel rod positions, the fuel rods maybe inclined.

FIG. 3 shows how the upper end of the fuel rods is displaced one or twosteps in the lattice in relation to the lower end of the fuel rods. Thedisplacement takes place within the same ring such that the fuel rodswill incline in the direction of tangent of the ring. In the inner ring14 and the middle ring 15, the fuel rods are displaced one step, and inthe outer ring 16, the fuel rods are displaced two steps. All the fuelrods in a rotary cell are inclined in the same direction, that is,either clockwise or anticlockwise around the centre of the rotary cell.The purpose of inclining the fuel rods around the centre of the rotarycell is to put the coolant into rotation, that is, the water and thesteam which flow upwards through the fuel assembly, thus achieving aneddy with a centre at the centre of the rotary cell. The eddy may bedirected clockwise or anticlockwise, depending on in which direction thefuel rods in the two rings are inclined.

The polar lattice gives optimum conditions for inclined rods as a meansof achieving or supporting the flow rotation. The patterns ofinclination become simple and refined. Problems with rods getting tooclose to each other during the inclination are avoided. The lattice isin this example completely rotationally symmetrical.

FIG. 2c shows an example of an approximately octagonal lattice geometry.The flow-guiding structure is adapted to the octagonal geometry. Thecentre tube 8 c is square with straight walls, the V-shaped guide bars 9c also have straight walls, and the corners 7 c of the fuel channel arecut off straight. An advantage of this embodiment compared with thepreviously described one is that it is simpler and hence less expensiveto manufacture.

The steam separation assumes that a volume is opened up around therotary centre of the eddy in the upper part of the fuel assembly. Inthis embodiment, the open volume is achieved by excluding one or morefuel rods in the inner ring in the upper fuel units. The fuel units inthe upper and lower parts of the fuel assembly have the same lattice butthe number of occupied fuel rod positions is different.

The fuel assembly comprises two different types of fuel units, of whichone type 3 a is intended to be arranged in the lower part of the fuelassembly and the other type 3 b is intended to be arranged in the upperpart of the fuel assembly. FIG. 2a shows a horizontal section A—Athrough a fuel unit 3 a in the lower part of the fuel assembly. In thefuel unit 3 a, all the fuel rod positions in the lattice are occupied byfuel rods. FIG. 2b shows a horizontal section B—B through a fuel unit 3b in the upper part of the fuel assembly. The fuel unit 3 b comprisesfour rotary cells, each having 24 fuel rods arranged in two rings withrespectively 8 and 16 fuel rods each in each ring. Four fuel rodpositions in the centre of the rotary cell are unoccupied. In this way,an empty volume is formed in the centre of the rotary cell. Otherwise,the fuel unit 3 b is arranged in the same way as the fuel unit 3 a andthe fuel rods in the upper 3 b and lower 3 a fuel units have the samediameter d₁. The empty volume constitutes a vertical steam conductingchannel which extends through the five uppermost fuel units in the fuelassembly. In the three lowermost fuel units 3 a, no steam conductingchannel is needed since there is not too much steam there. On the otherhand, it is an advantage to initiate the eddy formation as early aspossible.

There are four steam conducting channels 17 a, 17 b, 17 c, 17 d in thefuel assembly, one in each rotary cell. The inclined fuel rods in therotary cell brings about an eddy of water and steam around the steamconducting channel. The direction of the eddies are marked by arrows inthe steam conducting channel. In this eddy, the water and the steam areseparated from each other by the water being thrown outwards and henceaway from the steam conducting channel whereas the steam is pressedagainst the centre of the eddy. Because of the low density of the steamand the low flow resistance in the steam conducting channel, the steamwill flow upwards through the steam conducting channel at great speedand disappear out through the top of the fuel assembly. In this way, theproportion of steam by volume in the coolant is reduced.

The bottom tie plate and the top tie plate are provided with a largenumber of openings to allow the passage of coolant. To strengthen theeddies around the steam conducting channel, the bottom tie plate and thetop tie plate are preferably provided with rotary vanes. FIG. 8a showsan example of a bottom tie plate for a fuel unit 3 a in a section K—Kthrough FIG. 1. FIG. 8b shows in more detail the design of part of thebottom tie plate. Rotary vanes 18 are arranged to guide the water andthe steam in the direction of the eddy.

FIG. 4a shows an additional embodiment of the invention. In thisembodiment, a volume is opened up around the rotary centre in the upperpart of the assembly through a transformation of the lattice. The fuelunits 3 c in the upper part of the fuel assembly have a lattice whichdiffers from the lattice in the fuel units 3 a in the lower part of thefuel assembly in that at least certain lattice positions have beendisplaced outwards from the centre of the rings. This latticetransformation results in the fuel rods in the upper part of the fuelassembly being displaced outwards in a radial direction in relation tothe fuel rods in the lower part of the fuel assembly. In the same way asin the preceding example, the upper end of the fuel rods is displacedone or two steps in the lattice in relation to the lower end of the fuelrods. The displacement takes place within the same ring so that the fuelrods are inclined in the tangential direction of the ring.

FIG. 2a shows a fuel unit 3 a in the lower part of the fuel assembly,which has a first lattice with 28 fuel rod positions in each rotarycell. FIG. 4b shows a fuel unit 3 c in the upper part of the fuelassembly, which has a second lattice with 27 fuel rod positions in eachrotary cell. The fuel unit 3 c has four open volumes 25 a, 25 b, 25 c,25 d in the centre of each rotary cell.

FIG. 4d shows how the lattice positions 19 b in the second lattice aredisplaced in relation to the lattice positions 19 a in the firstlattice. Between these fuel units there is a third type of fuel unit, atransition unit 3 d, which comprises both the first and the secondlattice. The bottom tie plate has a lattice which is substantially thesame as the lattice in the lower fuel units 3 a and the top tie platehas a lattice which is substantially the same as the lattice in theupper fuel units 3 c. The fuel rods in the fuel unit 3 d are thusinclined outwards in a radial direction. In addition to providing anopen volume, also a transport of water along the outwardly inclined fuelrods towards the periphery of the fuel assembly is obtained in this way.The fuel rods in the fuel unit 3 d may, in the same way as in the otherunits, be arranged so as to be inclined in the direction of tangent ofthe ring. The fuel rods in the fuel unit 3 d will then be inclined bothradially and tangentially.

One advantage of inclining the fuel rods outwardly instead of omittingcentral fuel rods is the increased possibility of bringing water towardsthe peripheral parts of the rotary cell, since the water tends to followthe rods in the form of a film.

Another difference between the fuel units is that the fuel rods 20 inthe upper fuel units 3 c and the transition unit 3 d have a rod diameterd₂ which is smaller than the rod diameter d₁ of the fuel rods 4 in thelower units 3 a. This results in the free flow area being significantlysmaller in the lower part of the fuel assembly compared with the upperpart thereof. The greatly limited flow area in the lower part gives thewater a high flow rate and thus speeds up the rotation even from thebeginning. The narrower fuel rods prevent too high a pressure drop inthe upper part of the fuel assembly. To further increase the free flowarea in the upper part of the fuel assembly, the guide bar 21 has asmaller cross section in the upper part of the fuel assembly than in thelower part thereof.

FIG. 5 shows a further embodiment of the invention, in which the fuelunits 3 e in the upper and the fuel units 3 a in the lower part of thefuel assembly have different lattices but the same number of fuel rods.FIG. 2a shows the fuel unit 3 a in the lower part of the fuel assembly,which has a first lattice with 28 fuel rod positions in each rotarycell. FIG. 6 shows a section through the fuel unit 3 e in the upper partof the fuel assembly, which has a second lattice with 28 fuel rodpositions in each rotary cell.

FIG. 5a shows how the lattice positions 21 b in the second lattice aredisplaced in relation to the lattice positions 21 a in the firstlattice. To make room for all the fuel rods in the second lattice, thecentre tube 23 has a cross-section area in the upper part of the fuelassembly which is smaller than its cross-section area in the lower partof the fuel assembly. The fuel rods 20 in the upper fuel units 3 e havea rod diameter d₂ which is smaller than the rod diameter d₁ of the fuelrods 4 in the lower units 3 a. The fuel assembly may comprise one ormore transition units between the upper and lower fuel units. Toincrease the free flow area in the upper part of the fuel assembly, theguide bar 24 terminates below the upper fuel units.

FIG. 9 schematically shows a fuel assembly according to the invention,in which the bypass channels 9 b are provided with outflow holes 26 intheir upper end and with outer inflow holes 28 in the fuel channel andinner inflow holes 27 in the guide bar. The fuel assembly has a bottomsupport 29 with an inlet opening for each rotary cell. The bottomsupport may be provided with guide bars or oblique channels.

In these embodiments, the fuel units are retained over the whole crosssection of the fuel assembly. Of course, other embodiments may beprovided, with partitions which connect the walls of the fuel channel tothe centre tube.

What is claimed is:
 1. A fuel assembly for a boiling water reactor whichis adapted, during operation of the reactor, to allow coolant to flowupwards through the fuel assembly, and wherein the fuel assemblycomprises a plurality of fuel rods, each of which having an upper endand a lower end, a steam conducting channel through which steam flowsthrough the fuel assembly, and a fuel channel (7) surrounding the fuelrods, characterized in that the fuel assembly comprises a rotary cell(13 a-13 d) in which the fuel rods (4, 20) are arranged in a number ofconcentric rings (15, 16) with a substantially circular shape, and saidsteam conducting channel (17 a-17 d) is arranged in the centre of theconcentric rings, and that at least certain of the fuel rods in therings are arranged such that their upper ends are displaced in relationto their lower ends in a tangential direction such that the flowingcoolant is brought to rotate around the steam conducting channel.
 2. Afuel assembly according to claim 1, characterized in that all the fuelrods in the rotary cell are inclined in the same direction and thisdirection is either clockwise or anticlockwise around the steamconducting channel.
 3. A fuel assembly according to claim 1,characterized in that the fuel assembly comprises a plurality of rotarycells (13 a-13 d) and that each one of the rotary cells is at leastpartially surrounded by a flow-guiding structure (8 a, 9 a, 7 b).
 4. Afuel assembly according to claim 3, characterized in that at least partof the flow-guiding structure consists of a tubular member (8 a)arranged in the central part of the fuel assembly with its longitudinaldirection substantially parallel to the longitudinal direction of thefuel assembly.
 5. A fuel assembly according to claim 3, characterized inthat the flow-guiding structures comprise guide members (10) tomechanically strengthen and contribute to the rotation of coolant aroundthe steam conducting channels.
 6. A fuel assembly according to claim 3,characterized in that at least part of the flow-guiding structureconsists of an elongated member (9 a), the longitudinal direction ofwhich is substantially parallel to the longitudinal direction of thefuel assembly and which is arranged at the fuel channel (7).
 7. A fuelassembly according to claim 6, characterized in that said elongatedmember (9 a) and the fuel channel (7) together form a channel (9 b)which allows the passage of coolant.
 8. A fuel assembly according toclaim 1, characterized in that the fuel assembly comprises a pluralityof fuel units (3 a-3 e) stacked on top of each other, each onecomprising a top tie plate (5), a bottom tie plate (6), a plurality offuel rods (4, 20) extending between the top tie plate and the bottom tieplate, and that at least certain of the fuel units (3 b-3 e) comprise anempty volume which constitutes part of said steam conducting channel (17a-17 d, 25 a, 25 d).
 9. A fuel assembly according to claim 8,characterized in that it comprises a fuel unit (3 d) in which at leastcertain of the fuel rods are arranged such that the first end isdisplaced in a radial direction outwards from the centre of the ring inrelation to the lower end.
 10. A fuel assembly according to claim 8,characterized in that at least some fuel unit (3 a-3 e) has reducedcorner portions (7 b, 7 c) and that the corner portions are providedwith corner bars (12).
 11. A fuel assembly according to claim 8,characterized in that it comprises a lower fuel unit (3 a) with a firstlattice and an upper fuel unit (3 c, 3 e) with a second lattice whichdiffers from the first lattice in that at least some of the positions inthe lattice have been displaced outwards in a radial direction.
 12. Afuel assembly according to claim 11, characterized in that the majorityof the fuel rods (20) in the upper fuel unit (3 c, 3 e) have a diameter(d₁) which is smaller than the diameter (d₂) of the majority of the fuelrods (4) in the lower fuel unit (3 a).